As a method for transporting natural gas by converting the natural gas into a hydrate thereof, there has been proposed a transporting method in which the natural gas is converted into the hydrate thereof in production plant adjacent to the mining site, which hydrated natural gas, as the product, is put into a product storage container, and the product storage container is used as the transportation container to load on a transportation means such as, for example, a transport ship and to transport the hydrated natural gas to a consuming region, then said product storage container is used as the raw material storage container at a re-gasification plant adjacent to the consuming region, which allows the decomposition of dehydrated natural gas (for example, refer to Patent Document 1).
On transporting natural gas after being hydrated, however, the hydrate of natural gas, or gas hydrate, has a low filling rate in as powder state, (filling rate of 0.4, for example), and gives poor handling performance. Consequently, there are necessities to increase the filling rate and to increase the handling performance.
When a powdery gas hydrate is molded into pellets by using a granulation apparatus, the filling rate increases (filling rate of 0.56, for example). Since, however, the granulation apparatus is filled with a portion of non-reacted gas in the gas hydrate production apparatus, when the pellets formed by the granulation apparatus are carried out to a storage tank set under atmospheric pressure, the high-pressure non-reacted gas enters the storage tank together with the pellets. Thus, the storage tank is required to be fabricated to endure high pressure.
The storage tank expects the one having large capacity, such as a tank having 60 to 70 m in diameter and 20 to 30 m in height. When such large capacity storage tank is designed to pressure-resistant one, the cost becomes excessive, which causes loss of advantages of producing, storing, and transporting the gas hydrate of natural gas and water. Therefore, further technology innovation is required in order to store the pellets molded by a granulation apparatus in a storage tank under atmospheric pressure without accompanying high pressure non-reacted gas.
In addition, on storing pellets, when the pellets are charged from the upper part of the storage tank, they may collide with the bottom of the storage tank or with other pellets accumulated in the tank, which may break or disrupt pellets. Break or disruption of pellets deteriorates the self-retaining effect, and likely induces gasification. If pellet debris gets mixed into the slurry mother liquid, the slurry mother liquid becomes sherbet state, which makes the adjustment of pellet mixing rate difficult on transporting the pellets. When the pellets are discharged from a storage tank for loading on a ship, the carry-out of the pellets may become difficult as the pellets in the storage tank are consolidated.    Patent Document 1: Japanese Patent Application Kokai Publication No. 2001-280592